The present invention relates to electronic timing circuitry; and more particularly, to an electronic system capable of operating either as a timer (that is, decrementing time from a preset value) or a stopclock (that is, incrementing time, usually from zero). The stopclock mode is sometimes referred to as an elapsed time mode.
Briefly, the present invention uses a conventional circuit of the type used in electronic calculators to perform arithmetic and logical operations; hence it is referred to as a "calculator". It may include an input register for receiving and storing data temporarily and output registers which serve as accumulators for subsequently displaying either the preset time or the elapsed or remaining time (depending upon the mode). The outputs are displayed by means of Light Emitting Diode Arrays which are also conventional.
A keyboard enables an operator to enter a predetermined value of time into the calculator. A mode switch determines whether the system will operate in the timer mode or the stopclock mode. In the timer mode, an indication is given when a preset time has elapsed. The operator enters the preset time by means of the keyboard into the calculator, with the mode switch in the timer position. Next, the operator presses a "START/STOP" switch; the first actuation of the START/STOP switch causes the sequencing circuitry to begin operation. A subsequent actuation of the START/STOP switch will cause the timer to stop.
In the timer mode, the sequencing circuitry first effects an addition function which enters a preset time value. Next, the sequencing circuitry enters the value "1" which serves as the future decrement value. Next, the sequencing circuitry enables the passage of an output train of pulses from an accurate oscillator circuit, each pulse effecting a subtraction function to thereby decrement the preset time value accordingly until the preset time has elapsed. The system continues to decrement the time value into negative numbers to indicate elapsed time beyond expiration of the preset time.
Sensing circuitry senses the presence of a minus sign from the calculator in the "tenths" digit position, and generates an audio signal to alert the user. The START/STOP switch enables the user to both start and stop the system by triggering a latching circuit to change states upon successive closures of the switch.
In the stopclock mode, the system gives the user the capability of entering a preset time, although in this mode operation normally begins with zero time, since elapsed time is being measured. When the START/STOP switch is actuated a first time, the sequencing circuitry effects an addition function in the calculator to load the preset time (or the usual ZERO value if no preset time had been entered by the keyboard) into the calculator. A subsequent pulse from the sequencing circuitry establishes unity as the incrementing value, and thereafter, the sequencing circuitry couples the output of the oscillator to effect the addition function in the calculator to increment the stored value.
The oscillator is designed to generate an output pulse once every one-hundredth of a minute, so the timer has a nominal resolution of 0.01 minute. Since the oscillator has been found to be very accurate, the timing system is correspondingly accurate.
With the present invention, we have been able to provide a highly accurate, yet economical and reliable system, using modern electronic components and techniques developed for the computing arts. Because the system has a calculator and a keyboard, great flexiblity in use is provided. Further, greater accuracy and reliability are provided over conventional mechanical systems of equal costs.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing.